Method for producing polybutadiene latex with an optimized thermal current profile

ABSTRACT

A polybutadiene latex is prepared by a process in which butadiene is polymerized by free radical emulsion polymerization in the presence of initiator and, if required, dispersants and further conventional assistants, the polymerization being carried out in the presence of reactive comonomers in such a way that the heat flow of 43 watt/kg solids content of the polymerization mixture is not exceeded. Owing to the presence of reactive comonomers, the reaction rate at the beginning of the reaction is increased, with the result that an overall flatter heat flow profile is obtained. A preferred reactive comonomer is styrene. Preferably, the process is carried out by a semi-batch procedure in which
         (a) in a first stage, a portion of butadiene and at least one portion of reactive comonomer in the form of an aqueous emulsion are initially taken with a thermal polymerization initiator and, if required, dispersants and further conventional assistants and the polymerization is initiated,   (b) in a second stage, the remaining amount of butadiene and, if required, the remaining amount of reactive comonomer are added undiluted or in the form of an aqueous emulsion, if required with dispersants and, if required, further conventional assistants, as a feed.

The present invention relates to a process for the preparation ofpolybutadiene latex, the polybutadiene latex itself prepared in thismanner, a process for the preparation of a polybutadiene graftcopolymer, the graft copolymer itself, molding materials preparedtherefrom and the use of the graft copolymers and molding materials.

Polybutadiene rubber, which can be used as a base foracrylonitrile/butadiene/styrene graft copolymers, is usually prepared byemulsion polymerization in a semi-batch process. In a first stage, aportion of butadiene is initially taken with an initiator as an aqueousemulsion and is prepolymerized and, in a second stage, the remainingamount of butadiene is added undiluted or in the form of an aqueousemulsion as a feed. The disadvantage of this process is the low reactionrate at the beginning of the feed and consequently a disadvantageousoverall heat flow profile of the polymerization reaction. Whereas onlylittle heat evolution is observable with little instantaneous additionat the beginning of feed, the beat flow profile in the further course ofthe polymerization reaction has a pronounced peak.

This is undesirable, especially in large plants, since a large heat flowadversely affects the operational safety of the plant and large coolingcapacities have to be reserved for the reactor. In addition, withincreasing reactor size, the removal of heat via the reactor wallbecomes increasingly difficult owing to the decreasing surfacearea:volume ratio. Additional cooling capacity therefore has to beprovided through evaporative coolers or internal or external heatexchangers.

EP-A 0 834 518 describes a process and an apparatus for the preparationof homo- and copolymers by the emulsion polymerization method, in whichthe reaction mixture present in the form of an emulsion is passedthrough an external circulation leading from the reaction vessel andback to it and comprising a heat exchanger.

EP-A 0 486 262 describes an emulsion polymerization process in which thefeed rate is monitored by means of online reaction calorimetry so that aspecific heat flow is not exceeded.

Heat peaks in the course of the reaction can be avoided by increasingthe reaction rate at the beginning of the reaction, with the result thatthe heat flow profile of the reaction is flattened while the total heatof reaction remains constant.

EP-A 0 761 693 describes the use of redox initiator systems forincreasing the reaction rate at the beginning of the emulsionpolymerization. A monomer emulsion is initially taken and the initiatoris metered in together with further monomer.

It is an object of the present invention to provide a process for thepreparation of polybutadiene latex by emulsion polymerization, whichprocess is easy to carry out and in which peaks in the heat flow profileof the polymerization reaction are avoided.

We have found that this object is achieved by a process for thepreparation of polybutadiene latex, in which butadiene is polymerized bya free radical emulsion polymerization reaction in the presence ofinitiator and, if required, dispersants and further conventionalassistants, wherein the polymerization is carried out in the presence ofreactive comonomers in such a way that a heat flow of 43 watt/kg solidscontent of the polymerization mixture is not exceeded. Preferably, aheat flow of 39, particularly preferably of 34, watt/kg is not exceeded.

The heat flow profile of the polymerization can generally becharacterized as follows:

-   -   during a first period of 10-700, preferably 10-600, particularly        preferably 10-400, minutes, the change in the heat flow per unit        time is 0.01-0.5, preferably 0.01-0.4, watt/(kg·min);    -   during a second period of 0-600, preferably 100-500,        particularly preferably 100-400, minutes following the first        period, the change in the heat flow per unit time is −0.1-0.1,        preferably −0.05-0.05, particularly preferably −0.01-0.01,        watt/(kg·min);    -   during a third period of 10-1000, preferably 100-800, preferably        200-800, minutes following the second period, the change in the        heat flow per unit time is <0 watt/(kg·min).

Owing to the presence of reactive comonomers, the reaction rate at thebeginning of the reaction is increased, with the result that an overallflatter heat flow profile is achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows heat flow curves for Examples a to e according to theinvention.

FIG. 2 shows the corresponding conversion/time curves.

Suitable reactive comonomers are in general ethylenically unsaturatedmonomers, such as styrene, styrene derivatives substituted on thenucleus by alkyl or halogen, α-methylstyrene, alkyl acrylates, such asn-butyl acrylate, acrylonitrile, methacrylonitrile and mixtures thereof.A preferred reactive comonomer is styrene.

The novel process can be carried out continuously or by a semi-batchprocedure. Preferably, it is carried out by a semi-batch procedure inwhich

-   -   (a) in a first stage, a portion of butadiene and at least one        portion of reactive comonomer in the form of an aqueous emulsion        are initially taken with a thermal polymerization initiator and,        if required, dispersants and further conventional assistants and        the polymerization is initiated;    -   (b) in a second stage, the remaining amount of butadiene and, if        required, the remaining amount of reactive comonomer are added        undiluted or in the form of an aqueous emulsion, if required        with dispersants and, if required, further assistants, as a        feed.

Suitable thermal polymerization initiators for initiating thepolymerization reaction are all free radical formers which decompose atthe chosen reaction temperature, i.e. both those which decompose throughheat alone and redox initiators. Preferred initiators are free radicalformers which decompose through heat alone, for example peroxides, suchas sodium or potassium persulfate, and azo compounds, such asazobisisobutyronitrile. However, redox systems may also be used, inparticular those based on hydroperoxides, such as cumyl hydroperoxide.

In the first stage of the semi-batch process, a portion of butadiene andat least one portion of reactive comonomer in the form of an aqueousemulsion are initially taken, the total amount of reactive comonomerpreferably being initially taken. As a rule, conventional emulsifiersare present as dispersants, for example alkali metal salts ofalkanesulfonic or alkylarylsulfonic acids, alkylsulfates, fatty alcoholsulfonates, salts of higher fatty acids of 10 to 30 carbon atoms,sulfosuccinates, ether sulfonates or resin soaps. Preferred emulsifiersare the sodium and potassium salts of alkanesulfonic acids or fattyacids of 10-18 carbon atoms. The emulsifiers are usually used in amountsof from 0.5 to 5, preferably from 0.3 to 3, % by weight, based on themonomers used in the preparation of the polybutadiene latex.

Preferably, the proportion of the monomers (butadiene+reactivecomonomer) in the total amount of monomers in the initially takenmixture and feed is from 5 to 50, particularly preferably from 8 to 40,in particular from 10 to 30, % by weight. The feed can contain theremaining amount of butadiene and, if required, the remaining amount ofreactive comonomer undiluted or likewise in the form of an aqueousemulsion, if required with dispersants. Preferably, the feed containsbutadiene and the reactive comonomer in undiluted form. The feed isadded in general in the course of 1-18, preferably 2-16, particularlypreferably 2-12, hours. The monomer feed can be added in a plurality ofportions, batchwise or continuously, over the total period. Usually, thewater: monomer ratio is from 2:1 to 0.7:1. Initially taken mixture andfeed may contain further conventional assistants, such as molecularweight regulators or buffer substances. The dispersants and furtherassistants can also be added separately from the monomer feed, forexample batchwise in a plurality of portions, or continuously during aspecific time interval. Molecular weight regulators are, for example,ethylhexyl thioglycolate, n- or tert-dodecyl mercaptan, terpinols ordimeric α-methylstyrene. Buffer substances are, for example,Na₂HPO₄/NaH₂PO₄, sodium bicarbonate or citric acid/citrate buffer.

The total amount of reactive comonomer in the initially taken mixtureand, if required, in the feed is in general from 3 to 20, preferablyfrom 5 to 10, % by weight of the total amount of monomers.

Usually, polymerization is effected at from 20 to 100° C., preferablyfrom 30 to 80° C. The polymerization reaction is initiated by heatingthe polymerization mixture to the reaction temperature. In a preferredembodiment, the polymerization mixture initially taken in the firststage (a) is first prepolymerized after initiation of the reaction, andthe addition of the remaining amount of monomer (stage (b)) is not begununtil after a specific time. Usually, prepolymerization is effected for0.1-3 hours, preferably 0.25-1 hour. After the end of the feed,postpolymerization is usually carried out for a further period until thedesired monomer conversion has been reached. The polymerization reactionis preferably terminated on reaching a monomer conversion of about 90%.

The present invention also relates to a polybutadiene latex which can beprepared by one of the processes described above.

The present invention furthermore relates to a process for thepreparation of butadiene graft copolymers, in which a polybutadienelatex is prepared by one of the processes described above and

-   -   (c) in a further step, a graft layer comprising at least one        vinylaromatic monomer, acrylonitrile and, if required, further        ethylenically unsaturated monomers is grafted on.

In general, the graft layer is prepared from from 65 to 95, preferablyfrom 70 to 90, particularly preferably from 75 to 85, % by weight ofvinylaromatic monomer, from 5 to 35, preferably from 10 to 30,particularly preferably from 15 to 25, % by weight of acrylonitrile andfrom 0 to 30, preferably from 0 to 20, particularly preferably from 0 to15, % by weight of the further ethylenically unsaturated monomer.

Suitable vinylaromatic monomers are styrene and styrene derivatives,such as α-methylstyrene. Styrene is preferred. Further monomers are inparticular methyl methacrylate and acrylates, such as n-butyl acrylate,and N-phenylmaleimide. Methyl methacrylate in an amount of up to 20% byweight is very particularly preferred.

The graft layer can be prepared in one or more process steps. Forexample, in a two-stage grafting procedure, first styrene orα-methylstyrene alone and then styrene and acrylonitrile can bepolymerized in two successive steps. It is advantageous to carry out thepolymerization onto the grafting base once again in aqueous emulsion. Itcan be effected in the same system as the polymerization of the graftingbase, it being possible to add further emulsifier and initiator. Theseneed not be identical to the emulsifiers and initiators used for thepreparation of the grafting base. The monomer mixture to be grafted oncan be added to the reaction mixture all at once, batchwise in aplurality of stages or, preferably, continuously during thepolymerization.

The graft polymerization can furthermore be carried out in the presenceor in the absence of a molecular weight regulator, such as one of theabove-mentioned molecular weight regulators.

In general, the butadiene graft copolymers are prepared from from 40 to90% by weight of the novel polybutadiene latex and from 10 to 60% byweight of the monomers forming the graft layer.

The present invention also relates to butadiene graft copolymers whichcan be prepared by the process described above.

The novel butadiene graft copolymers can be mixed with thermoplasticcopolymers comprising at least one vinylaromatic monomer, acrylonitrileand, if required, further ethylenically unsaturated monomers, such ascommercial SAN polymers, and, if required, further thermoplasticpolymers and conventional additives to give thermoplastic moldingmaterials. Preferred molding materials contain from 5 to 80% by weightof butadiene graft copolymers and from 20 to 95% by weight of thethermoplastic copolymers, for example PSAN (polystyrene/acrylonitrilecopolymer).

The novel butadiene graft copolymers and thermoplastic molding materialscontaining them can be processed by the known methods for processingthermoplastics, such as extrusion, injection molding, calendering, blowmolding, compression molding or sintering.

The present invention also relates to the use of the butadiene graftcopolymers and of the molding materials containing them for theproduction of shaped articles, films and fibers by extrusion, injectionmolding, calendering, blow molding, compression molding or sintering.

The Examples which follow illustrate the invention.

EXAMPLES Preparation of the Butadiene Base Comparative Examples a and b

(Preparation of the Butadiene Base Without Styrene as Comonomer)

66.14 liters of demineralized water, 339 g of potassium stearate, 177.0g of sodium bicarbonate and 124.5 g of potassium persulfate wereinitially taken in a 160 l autoclave and heated to 67° C. 13.08 kg ofbutadiene were then added in the course of 30 minutes. 10 minutes afterthe beginning of this feed, 174.2 g of tert-dodecyl mercaptan wereadded. After the end of the first butadiene feed (initially takenmonomer), prepolymerization was effected for 30 minutes. 39.24 kg ofbutadiene were then added in the course of 9 hours. 4 hours after thebeginning of this main feed, 174.2 g of dodecyl mercaptan were added. 8hours after the beginning of this main feed, a further 174.2 g ofdodecyl mercaptan were added. After the end of the main feed,postpolymerization was carried out until the conversion reached 90%.Thereafter, the pressure was let down, cooling to 50° C. was effectedand the remaining butadiene was removed under reduced pressure. Thedispersion was then discharged into a drum.

Example b is the repetition of Example a.

Example c

(7% by Weight, Based on the Total Amount of Monomers in the InitiallyTaken Mixture and Feed, of Styrene in the Initially Taken Monomer)

66.14 liters of demineralized water, 339 g of potassium stearate, 177.0g of sodium bicarbonate and 124.5 g of potassium persulfate wereinitially taken in a 160 l autoclave and heated to 67° C. 9.42 kg ofbutadiene and 3.66 kg of styrene were then added in the course of 30minutes. 10 minutes after the beginning of this feed, 174.2 g oftert-dodecyl mercaptan were added. After the end of the first butadienefeed (initially taken monomer), prepolymerization was effected for 30minutes. 39.24 kg of butadiene were then added in the course of 9 hours.4 hours after the beginning of this main feed, 174.2 g of dodecylmercaptan were added. 8 hours after the beginning of this main feed, afurther 174.2 g of dodecyl mercaptan were added. After the end of themain feed, postpolymerization was carried out until the conversionreached 90%. Thereafter, the pressure was let down, cooling to 50° C.was effected and the remaining butadiene was removed under reducedpressure. The dispersion was then discharged into a drum.

Example d

(3.5% by Weight of Styrene in the Initially Taken Monomer, 3.5% byWeight in the Feed, Based in Each Case on the Total Amount of Monomersin the Initially Taken Mixture and Feed)

66.14 liters of demineralized water, 339 g of potassium stearate, 177.0g of sodium bicarbonate and 124.5 g of potassium persulfate wereinitially taken in a 160 l autoclave and heated to 67° C. 11.25 kg ofbutadiene and 1.83 kg of styrene were then added in the course of 30minutes. 10 minutes after the beginning of this feed, 174.2 g oftert-dodecyl mercaptan were added. After the end of the first butadienefeed (initially taken monomer), prepolymerization was effected for 30minutes. 37.41 kg of butadiene and 1.83 kg of styrene were then added inthe course of 9 hours. 4 hours after the beginning of this main feed,174.2 g of dodecyl mercaptan were added. 8 hours after the beginning ofthis main feed, a further 174.2 g of dodecyl mercaptan were added. Afterthe end of the main feed, postpolymerization was carried out until theconversion reached 90%. Thereafter, the pressure was let down, coolingto 50° C. was effected and the remaining butadiene was removed underreduced pressure. The dispersion was then discharged into a drum.

Example e

(10% by Weight, Based on the Total Amount of Monomers in the InitiallyTaken Mixture and Feed, of Styrene in the Initially Taken MonomerMixture)

66.14 liters of demineralized water, 339 g of potassium stearate, 177.0g of sodium bicarbonate and 124.5 g of potassium persulfate wereinitially taken in a 160 l autoclave and heated to 67° C. 7.85 kg ofbutadiene and 5.32 kg of styrene were then added in the course of 30minutes. 10 minutes after the beginning of this feed, 174.2 g oftert-dodecyl mercaptan were added. After the end of the first butadienefeed (initially taken monomer), prepolymerization was effected for 30minutes. 39.24 kg of butadiene were then added in the course of 9 hours.4 hours after the beginning of this main feed, 174.2 g of dodecylmercaptan were added. 8 hours after the beginning of this main feed, afurther 174.2 g of dodecyl mercaptan were added. After the end of themain feed, postpolymerization was carried out until the conversionreached 90%. Thereafter, the pressure was let down, cooling to 50° C.was effected and the remaining butadiene was removed under reducedpressure. The dispersion was then discharged into a drum.

Examples a to e were repeated with correspondingly reduced amounts in a2 liter heat flow calorimeter, the heat flow profile of thepolymerization reactions being measured.

Table 1 shows the results of the experiments.

The heat flow curves are shown in FIG. 1. With increasing styrenecontent of the initially taken mixture, a substantially flatter heatflow profile is obtained.

FIG. 2 shows the corresponding conversion/time curves.

Preparation of ABS Graft Copolymers and Blending with PSAN(Polystyrene/Acrylonitrile Copolymer) to Give ABS Molding Materials

Graft Variant 1

Comparative Example (CE) f and Examples g-j

4427.8 g of a polybutadiene latex, prepared according to Examples a-e,were introduced into a stirred 10 l flask and heated to 75° C. Thesolids content of the polybutadiene latex was 43.8% by weight. Duringthe heating-up, 387.9 g of a 10% strength by weight dispersion ofpoly(ethyl acrylate-co-methacrylamide) were added at 65° C. Thereafter,198.1 g of a mixture of 80 parts by weight of styrene and 20 parts byweight of acrylonitrile were added and prepolymerization was effectedfor 15 minutes. 990.5 g of the same monomer mixture were then added inthe course of 3 hours. Two hours after the beginning of this feed, thetemperature was increased to 80° C. After the end of the feed, 2.38 g ofpotassium persulfate were added and postpolymerization was carried outfor 1.5 hours. After cooling, a dispersion of an antioxidant was addedto the dispersion.

Graft Variant 2

Comparative Example k and Examples l-o

4427.8 g of a polybutadiene latex, prepared according to Examples a-e,and 5.9 g of tert-dodecyl mercaptan were introduced into a stirred 10 lflask and heated to 75° C. The solids content of the polybutadiene latexwas 43.8% by weight. During the heating-up, 387.9 g of a 10% strength byweight dispersion of poly(ethyl acrylate-co-methacrylamide) were addedat 65° C. Thereafter, 198.1 g of a mixture of 80 parts by weight ofstyrene and 20 parts by weight of acrylonitrile were added andprepolymerization was effected for 15 minutes. 990.5 g of the samemonomer mixture were then added in the course of 3 hours. Two hoursafter the beginning of this feed, the temperature was increased to 80°C. After the end of the feed, 2.38 g of potassium persulfate were addedand postpolymerization was carried out for 1.5 hours. After cooling, adispersion of an antioxidant was added to the dispersion.

The properties of the rubber are shown in Tables 2 and 4.

The polybutadiene dispersions grafted according to graft variants 1 and2 were precipitated in MgSO₄ solution and compounded with 70% by weightof PSAN to give ABS molding materials.

The properties of the molding materials are shown in Table 3.

In all the Tables below, the meanings are as follows:

SC: Solids content [%] t(90% conversion): Time taken to reach 90%conversion [h] Q_(max): Maximum heat flow [watt/kg solids content] SI:Swelling index Gel: Gel content [%]; the gel content indicates thecrosslinked fraction of the rubber particle. d₅₀: Weight averageparticle diameter [nm]; indicates the particle diameter at which 50% byweight of all particles have a larger particle diameter and 50% byweight of all particles have a smaller particle diameter. (d₉₀-d₁₀)/d₅₀:The d₁₀ value indicates the particle diameter at which 10% by weight ofall particles have a smaller diameter and 90% by weight have a largerdiameter. Accordingly, the d₉₀ value indicates the particle diameter atwhich 90% by weight of all particles have a smaller diameter and 10% byweight have a larger diameter. The quotient (d₉₀-d₁₀)/d₅₀ characterizesthe width of the particle size distribution. The smaller this value, thenarrower is the distribution. a_(K): Charpy notched impact strength[kJ/m²]; this was determined using standard small bars by the impactbending test according to ISO 179-2/1 eA (S) at room temperature (RT)and −40° C. Vicat B: Vicat B temperature [° C.]; the heat distortionresistance according to Vicat was determined using small pressed sheetsaccording to 150306/B with a load of 50 N and a heating rate of 50 K/h.a_(D): Total penetration energy; this was determined according to ISO6603-2 using round disks or 40-40 mm rectangular panels by thePlastechon test at 23° C. T_(g): Glass transition temperature [° C.].MVR: Melt volume rate according to DIN 53735/30 at 220° C. and 10 kgload

TABLE 1 Example a(CE) c d e SC [%] 44.1 43.9 43.3 43.7 t(90% conversion)[h] 21.5 15.3 17.8 15.3 Q_(max) [watt/kg solids content] 44 38 43 32 SI20 21 34 20 Gel [%] 80 78 69 78 d₅₀ [nm] 83 105 90 111 (d₉₀-d₁₀)d₅₀0.530 0.457 0.589 0.387

TABLE 2 Graft Example variant SI Gel [%] d₅₀ [nm] (d₉₀-d₁₀)/d₅₀ f(CE) 19.38 87.99 130 2.886 k(CE) 2 12.86 78.45 124 3.030 h 1 12.03 80.27 1702.132 m 2 15.65 70.28 149 2.501 i 1 11.06 83.58 174 2.150 n 2 15.7672.41 156 2.470 j 1 10.43 85.51 197 1.934 o 2 12.4 79.44 179 2.348

TABLE 3 MVR (220/10) Graft a_(k)(RT) a_(K)[−40° C.] a_(D) Vicat B [ml/Example variant [kJ/m²] [kJ/m²] [Nm] [° C.] 10 min] f(CE) 1 22.5 10.619.6 99.7 12.0 k(CE) 2 28.2 8.5 25.1 98.5 13.0 h 1 19.7 9.8 23.7 97.612.3 m 2 26.6 10.6 25.2 98.3 13.9 i 1 18.3 8.1 20.0 97.7 12.6 n 2 30.37.7 29.7 97.3 14.4 j 1 19.6 7.4 28.8 97.7 11.8 o 2 25.4 8.1 32.3 97.514.6

The mechanical properties of the blends are at a constant level.

TABLE 4 Example Graft variant T_(g)[° C.] f(CE) 1 −88 h 1 −83 i 1 −80 j1 −78

The glass transition temperature of the rubber increases onlydisproportionately relative to the styrene content, which indicates aheterogeneous structure of the graft rubber particles.

1. A process for the preparation of polybutadiene latex, in whichbutadiene is polymerized by a free radical emulsion polymerizationreaction in the presence of initiator and, if required, dispersants andfurther conventional assistants, wherein the polymerization is carriedout in the presence of reactive comonomers in such a way that a beatflow of 43 watt/kg solids content of the polymerization mixture is notexceeded further comprising a heat flow profile with: during a firstperiod of 10-700 minutes, a change in the heat flow per unit time of0.01-0.5 watt/(kg·min); during a subsequent second period of 0-600minutes, a change in the heat flow per unit time of −0.1-0.1watt/(kg·min); during a subsequent third period of 10-1000 min, a changein the heat flow per unit time of <0 watt/(kg·min), wherein thepolymerization is carried out by a semi-batch procedure in which (a) ina first stage, a portion of butadiene and at least one portion ofreactive comonomer in the form of an aqueous emulsion are initiallytaken with a thermal polymerization initiator and, if required,dispersants and further conventional assistants and the polymerizationis initiated, (b) in a second stage, the remaining amount of butadieneand, if required, the remaining amount of reactive comonomer are addedundiluted or in the form of an aqueous emulsion, if required withdispersants and, if required, further conventional assistants, as afeed.
 2. A process for the preparation of polybutadiene latex, in whichbutadiene is polymerized by a free radical emulsion polymerizationreaction in the presence of initiator and, if required, dispersants andfurther conventional assistants, wherein the polymerization is carriedout in the presence of reactive comonomers in such a way that a heatflow of 43 watt/kg solids content of the polymerization mixture is notexceeded, further comprising a heat flow profile with: during a firstperiod of 10-700 minutes, a change in the heat flow per unit time of0.01-0.5 watt/(kg·min); during a subsequent second period of 0-600minutes, a change in the heat flow per unit time of −0.1-0.1watt/(kg·min); during a subsequent third period of 10-1000 min, a changein the heat flow per unit time of <0 watt/(kg·min).
 3. A process asclaimed in claim 1, wherein the total amount of reactive comonomer isinitially taken.
 4. A process as claimed in claim 1, wherein the totalamount of reactive comonomer is from 3 to 20% by weight of the totalamount of monomers (butadiene+reactive comonomer).
 5. A process asclaimed in claim 1, wherein the reactive comonomer is styrene.
 6. Aprocess for the preparation of butadiene graft copolymers, in which apolybutadiene latex is prepared in the form of a grafting base by aprocess as claimed in claim 1 and (c) in a further step, a graft layercomprising at least one vinylaromatic monomer, acrylonitrile and, ifrequired, further ethylenically unsaturated monomers is grafted on.
 7. Aprocess as claimed in claim 6, wherein step (c) is carried out in thepresence or in the absence of a molecular weight regulator.
 8. A processas claimed in claim 2, wherein the polymerization is carried out by asemi-batch procedure in which (a) in a first stage, a portion ofbutadiene and at least one portion of reactive comonomer in the form ofan aqueous emulsion are initially taken with a thermal polymerizationinitiator and, if required, dispersants and further conventionalassistants and the polymerization is initiated, (b) in a second stage,the remaining amount of butadiene and, if required, the remaining amountof reactive comonomer are added undiluted or in the form of an aqueousemulsion, if required with dispersants and, if required, furtherconventional assistants, as a feed.
 9. A process as claimed in claim 2,wherein the total amount of reactive comonomer is initially taken.
 10. Aprocess as claimed in claim 2, wherein the total amount of reactivecomonomer is from 3 to 20% by weight of the total amount of monomers(butadiene+reactive comonomer).
 11. A process as claimed in claim 2,wherein the reactive comonomer is styrene.
 12. A process for thepreparation of butadiene graft copolymers, in which a polybutadienelatex is prepared in the form of a grafting base by a process as claimedin claim 8 and (c) in a further step, a graft layer comprising at leastone vinylaromatic monomer, acrylonitrile and, if required, furtherethylenically unsaturated monomers is grafted on.
 13. A process asclaimed in claim 12, wherein step (c) is carried out in the presence orin the absence of a molecular weight regulator.